1. Field of the Invention
The present invention relates to an optical element having the antireflection function and an optical apparatus including the optical element.
2. Description of the Related Art
Conventionally, as antireflection structure on the surface of an optical system, along with an antireflection film using the interference in dielectric multilayer films, an antireflection structure using fine relief structure of at most the wavelength is known. This utilizes a characteristic where light reacts in the fine relief structure of at most the wavelength as layers having homogenous effective refractive index. The effective refractive index for the fine relief structure or porous structure is regarded as an average refractive index which includes refractive indexes of medium and air.
Therefore, if the fine relief structure is constituted so that the density of the medium lowers form the substrate interface toward air, the effective refractive index gradually lowers from a substrate interface toward the air. This inclination of the refractive index drastically reduces the reflection on the interface.
However, the antireflection structure using the fine relief structure has a weak point where the manufacture is difficult by the fineness thereof.
For example, Japanese Patent Laid-Open No. 2001-272505 discloses a surface treatment method of forming a taper shape having a large aspect ratio for the antireflection on an optical element.
This method forms a metallic mask having a dot array shape for the optical substrate and etches the optical substrate itself, and in the etching, gradually reduces the diameter of the metallic mask to form the taper shape. Japanese Patent Laid-Open No. 2001-272505 further discloses that a similar processing is given on a metal mold, and an optical element is formed by the metal mold.
On the other hand, Japanese Patent Laid-Open No. 2005-70435 has a shape similar to the present invention, and discloses a structure where reliefs are formed on a surface of a substrate, and antireflection multilayer film is formed on the reliefs. The manufacture thereof seems to be easy because the reliefs of the surface of the substrate have a shallow depth compared with the pitch, and its aspect ratio is small.
However, the technical art disclosed in Japanese Patent Laid-Open No. 2001-272505 uses complex processes, such as the formation of the metallic mask and the etching, in the manufacture of the relief structure on the substrate, and therefore is inept in quantity production for consumer productions in a cost. The molding is difficult because the aspect ratio is high, and the increase of the molding takt and the lowering of non-defective product rate may be caused.
On the other hand, in Japanese Patent Laid-Open No. 2005-70435, since the pitch of the reliefs is 2˜100 μm more than the wavelength, the substrate is different from the fine relief shape and cannot be treated as homogeneous effective refractive index in the surface even by approximation. Since the relief shape is sufficiently large as compared to the wavelength, the diffractive scattering is caused on the surface. In the first place, the conventional art purposes the antiglare, its range of use is limited to proximity of a display surface of an monitor apparatus and the like, and therefore it cannot be used in, for example, a surface of an image-pickup lens in an imaging system. The thin film formed in the relief shape merely has the performance as a normal multilayer film, and the improvement of the angle characteristic is not expected